The equation of state of the prefect fluid that models the Friedmann universe is $$

p=w\rho

$$where ##p,\rho## are pressure and energy density and ##w## is a constant. In section 4.6 Carroll describes five 'popular' energy conditions on the energy-momentum tensor without saying why they are popular or even why they are interesting and shows that for any of them to be true we must have ##w\geq-1## in the equation of state. The energy conditions, which sound like characters in a robot movie, are WEC, NEC, DEC, NDEC and SEC. Possible values of ##\rho,p## are shown in the diagram below for these energy conditions along with possible values when ##w\geq-1## in the equation of state. Assuming that our Friedmann universe has ##\rho>0## (highly plausible) and satisfies one of the robots (haven't got the faintest idea) then we must have ##w\geq-1##.

p=w\rho

$$where ##p,\rho## are pressure and energy density and ##w## is a constant. In section 4.6 Carroll describes five 'popular' energy conditions on the energy-momentum tensor without saying why they are popular or even why they are interesting and shows that for any of them to be true we must have ##w\geq-1## in the equation of state. The energy conditions, which sound like characters in a robot movie, are WEC, NEC, DEC, NDEC and SEC. Possible values of ##\rho,p## are shown in the diagram below for these energy conditions along with possible values when ##w\geq-1## in the equation of state. Assuming that our Friedmann universe has ##\rho>0## (highly plausible) and satisfies one of the robots (haven't got the faintest idea) then we must have ##w\geq-1##.

From the equation of state ##w\geq-1## gives$$

\frac{p}{\rho}\geq-1\Rightarrow p\geq-\rho\Rightarrow p+\rho\geq0

$$which is the same as the NEC. So why aren't the pictures (b) and (f) the same? I hope you've seen my error. Luckily I did and have avoided falsely accusing Carroll of a slip up.

\frac{p}{\rho}\geq-1\Rightarrow p\geq-\rho\Rightarrow p+\rho\geq0

$$which is the same as the NEC. So why aren't the pictures (b) and (f) the same? I hope you've seen my error. Luckily I did and have avoided falsely accusing Carroll of a slip up.

Carroll's picture with my adornments. |

Here is my limited understanding: Commentary 4.6 Energy conditions.pdf (3 pages)

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